Hollow Spherical Particles Research Articles

Synergistically enhanced electromagnetic absorption in barium ferrite/hollow carbon spheres/epoxy composites

The ongoing technological advancements have emphasized the importance of absorbent coatings, which are essential for effectively absorbing electromagnetic waves across various industries, including electronics, healthcare, and security systems. In this research, barium ferrite particles and hollow spherical carbon particles have been synthesized to investigate the synergistic effect of different compounds in absorbing electromagnetic waves in the epoxy coating. The barium ferrite particles were synthesized in two distinctive rod and spherical morphologies at 600 and 900 °C, employing the sol-gel technique. Also, hollow spherical carbon particles were synthesized. Synthesized particles were analyzed structurally and chemically with an X-ray Powder Diffraction, Fourier infrared spectrometer, vibrating‐sample magnetometer, field emission Scanning Electron, and Raman spectroscopy.The absorption characteristics of a coating that contains rod and spherical morphologies of barium ferrite and hollow spherical carbon particles can be adjusted by controlling the amounts of synthetic compounds in the 8–12 GHz range. The results indicate that combining rod-shaped barium ferrite particles (900 °C) and hollow spherical carbon particles in a thin epoxy coating improves impedance matching and wave attenuation. Notably, when the epoxy coating is 1.5–2 mm thick and contains 20 % rod-shaped barium ferrite particles (900 °C) and 1 % hollow spherical carbon particles, reflection loss values of < -20 dB are observed at frequencies of 10–12 GHz.

Assembly of Hollow Yttrium Oxide Spheres from Nano-Sized Yttrium Oxide for Advanced Passive Radiative Cooling Materials.

This study presents significant advancements in passive radiative cooling (PRC), achieved using assembled hollow yttrium oxide spherical particles (AHYOSPs). We developed PRC films with enhanced optical properties by synthesizing micro-sized hollow Y2O3 particles and integrating them into a polydimethylsiloxane (PDMS) matrix. The findings revealed that AHYOSPs achieved a remarkable solar reflectance of 73.72% and an emissivity of 91.75%, significantly outperforming nano-sized yttrium oxide (NYO) and baseline PDMS. Field tests demonstrated that the AHYOSPs maintained their lowest temperature during daylight, confirming their superior cooling efficiency. Additionally, theoretical calculations using MATLAB indicated that the cooling capacity of AHYOSPs reached 103.77 W/m2, representing a substantial improvement over NYO and robustly validating the proposed nanoparticle assembly strategy. These results highlight the potential of structurally controlled particles to revolutionize PRC technologies, thereby offering a path toward more energy-efficient and environmentally friendly cooling solutions.

Synthesis of Air@Co@Co7Fe3@Fe3O4 composite with enhanced electromagnetic wave absorption performance

Electromagnetic pollution prevention and military stealth technology requirements for wave absorbing materials gradually increased, therefore, the research and development of absorbing materials with the merits of low reflection loss, wide effective absorption bandwidth, and lightweight is of great significance. In this work, Air@Co@Co7Fe3@Fe3O4 hollow spherical particles have been successfully synthesized via a simple chemical plating method combined with a post heat treatment. It reveals that the Air@Co@Co7Fe3@Fe3O4 particles with a loading of 50 wt % exhibits an optimum reflection loss (RL) value of −20.77 dB at 15.52 GHz and broad effective absorption bandwidth (EAB, RL ≤ −10 dB) of 4.56 GHz with a thickness of 1.4 mm. Compared to Air@Co hollow spherical particles, Air@Co@Co7Fe3@Fe3O4 particles with natural resonance and eddy current loss, as well as the interlayer interfacial polarization mechanism of the multilayer structure, exhibit superior electromagnetic wave(EMW) absorption capabilities. The Air@Co@Co7Fe3@Fe3O4 hollow spherical particles could be a new highly efficient absorbing material for electromagnetic waves.

Sporopollenin Capsules as Biomimetic Templates for the Synthesis of Hydroxyapatite and β-TCP.

Pollen grains, with their resilient sporopollenin exine and defined morphologies, have been explored as bio-templates for the synthesis of calcium phosphate minerals, particularly hydroxyapatite (HAp) and β-tricalcium phosphate (TCP). Various pollen morphologies from different plant species (black alder, dandelion, lamb's quarters, ragweed, and stargazer lily) were evaluated. Pollen grains underwent acid washing to remove allergenic material and facilitate subsequent calcification. Ragweed and lamb's quarter pollen grains were chosen as templates for calcium phosphate salts deposition due to their distinct morphologies. The calcification process yielded well-defined spherical hollow particles. The washing step, intended to reduce the protein content, did not significantly affect the final product; thus, justifying the removal of this low-yield step from the synthesis process. Characterisation techniques, including X-ray diffraction, scanning electron microscopy, Fourier-transform infrared spectroscopy, and thermal gravimetric analysis, confirmed the successful calcification of pollen-derived materials, revealing that calcified grains were principally composed of calcium deficient HAp. After calcination, biphasic calcium phosphate composed of HAp and TPC was obtained. This study demonstrated the feasibility of using pollen grains as green and sustainable bio-templates for synthesizing biomaterials with controlled morphology, showcasing their potential in biomedical applications such as drug delivery and bone regeneration.

Self‐templated magnesiothermic reduction of ORMOSIL particles to monodisperse hollow spherical SiC composite particles

AbstractHollow silicon carbide (SiC) particles have attracted attention due to their well‐defined morphology, low density, and large surface area that can be leveraged for a wide variety of applications. Here, we report the self‐templated synthesis of monodisperse hollow spherical SiC composite particles with controllable shell thicknesses using magnesiothermic reduction of organically modified silica (ORMOSIL) particles. Monodisperse ORMOSIL particles containing three organic groups of methyl, vinyl, and mercaptopropyl were produced by a simple one‐pot sol–gel process and then converted to hollow spherical SiC composite particles via a magnesiothermic reduction at 700°C under an inert atmosphere. The spherical morphologies of the original ORMOSIL particles were well maintained through this self‐templated conversion process, but the sizes of the hollow SiC particles were slightly reduced by the thermal treatment. Field emission transmission electron microscopy studies revealed that the shell of hollow particles mainly consists of primary β‐SiC particles with nanometer‐scale sizes. The physical and chemical properties of the hollow SiC composite particles were investigated by scanning electron microscopy, transmission electron microscopy, X‐ray photoelectron spectroscopy, Brunauer–Emmett–Teller, X‐ray diffraction, Fourier transform infrared spectroscopy, Raman, and solid‐state nuclear magnetic resonance analyses. The shell thickness of hollow spherical SiC composite particles can be controlled by simply changing the duration of the thermal conversion process. This study not only provides a simple and easy approach for the preparation of hollow spherical SiC particles but also opens the possibility of large‐scale manufacturing of such particles for commercial applications.

Enhanced heating efficiency for hollow Fe3O4 spherical submicron particles

We have investigated ac hysteresis loops of hollow Fe3O4 submicron particles with variable particle size of d = 100−696 nm by micromagnetic simulations to investigate the possible application to magnetic hyperthermia. For the hollow particle with the inner/outer diameter ratio of γ = 0.5, the hysteresis loss increases with increasing d and maximizes at d ∼ 300 nm, whereas the hysteresis loss generally increases with γ, but its behavior strongly depends on d. A specific absorption rate, calculated from the loop area at the field frequency of 500 kHz, attains 560 W/g for d = 296 nm and γ = 0.5, which is comparable to that for conventional superparamagnetic nanoparticles. This enhanced specific absorption ratio for the hollow particles can be explained by strong irreversibility between vortex states with different orientation of the vortex core, i.e. along the magnetic field and ⟨111⟩ easy axes.

Preparation of gradient refractive index films on glass surface and its anti-reflection properties

This study presents the creation of a gradient refractive index film on the surface of aluminosilicate glass using a two-step process involving chemical etching and subsequent multilayer sol-gel coating with hollow silica nanospheres. The primary objective was to achieve effective anti-reflection properties. Various analytical techniques such as FESEM, TEM, UV-Vis-NIR spectrophotometry, ellipsometer, and FT-IR spectrometry were employed to investigate the impact of glass surface morphology and refractive index on optical characteristics. The results revealed the successful formation of a porous structure on the glass surface through chemical etching. Subsequently, this porous structure was uniformly filled and layered with hollow silica spheres of different sizes, resulting in a stepwise gradient refractive index coating. Notably, the pore size of the hollow spherical silica colloidal particles progressively increased from 0 nm to 23 nm, corresponding to distinct film thicknesses and refractive indices. This stepwise modulation contributed to the establishment of a gradient refractive index within the coating. By transitioning the refractive index of the film layer from that of glass to air, the film acted as a matching layer, thereby achieving broadband anti-reflection. The efficiency of this anti-reflection was demonstrated by an increase in transmittance from an initial value of 91.8–95.32% following the two-step etching process. With the subsequent SiO2 coating, the transmittance continued to improve, ultimately reaching a high value of 97.47%. This improvement in transmittance was attributed to the gradual enhancement of the film layer.

Characterization of Silicate Glass/Mullite Composites Based on Coal Fly Ash Cenospheres as Effective Gas Separation Membranes

Membrane technology is a promising method for gas separation. Due to its low energy consumption, environmental safety, and ease of operation, membrane separation has a distinct advantage over the cryogenic distillation conventionally used to capture light inert gases. For efficient gas recovery and purification, membrane materials should be highly selective, highly permeable, thermally stable, and low-cost. Currently, many studies are focused on the development of high-tech materials with specific properties using industrial waste. One of the promising waste products that can be recycled into membrane materials with improved microstructure is cenospheres—hollow aluminosilicate spherical particles that are formed in fly ash from coal combustion during power generation. For this purpose, based on narrow fractions of fly ash cenospheres containing single-ring and network structure globules, silicate glass/mullite composites were prepared, characterized, and tested for helium–neon mixture separation. The results indicate that the fragmented structure of the cenosphere shells with areas enriched in SiO2 without modifier oxides, formed due to the crystallization of defective phases of mullite, quartz, cristobalite, and anorthite, significantly facilitates the gas transport process. The permeability coefficients He and Ne exceed similar values for silicate glasses; the selectivity corresponds to a high level even at a high temperature: αHe/Ne—22 and 174 at 280 °C.

Emerging trends in the recovery of ferrospheres and plerospheres from coal fly ash waste and their emerging applications in environmental cleanup

Coal fly ash (CFA) is a major global problem due to its production in huge volumes. Fly ash has numerous toxic heavy metals; thus, it is considered a hazardous material. However, it also has several value-added minerals like ferrous, alumina, and silica along with other minerals. Fly ash also has several natural micro- to nano-structured materials; for instance, spherical ferrous-rich particles, cenospheres, plerospheres, carbon nanomaterials, and unburned soot. These micron- to nano-sized particles are formed from the molten slag of coal, followed by condensation. Among these particles, plerospheres which are hollow spherical particles, and ferrospheres which are ferrous-rich particles, have potential applications in the environmental cleanup, research, catalytic industries, and glass and ceramics industries. Additionally, these particles could be further surface-functionalized or purified for other applications. Moreover, these particles are widely explored for their potential in the army and other defense systems like lightweight materials and sensing The recovery of such particles from waste fly ash will make the process and remediation technology economically and environmentally friendly. The current review focuses on the various structural and elemental properties of ferrospheres and plerospheres from fly ash. This review also focuses on the emerging applications of both naturally formed materials in CFA.

Development and research of syntactic foams for antenna devices parts manufacture

Foam plastics are always widely used in antenna-feeder systems due to the low values of the dielectric constant ε and the tangent of the dielectric loss angle tgδ. Structural elements of such antennas are usually made by gluing blanks cut from expanded polystyrene plates or from filling polyurethane foam. However, these methods turned out to be low-tech due to the high labor intensity of the process, the inhomogeneity of the dielectric properties of structures and the unsatisfactory quality of the painted surface due to the low chemical resistance of these materials to solvents of paint and varnish materials. Syntactic foams, which are a special type of gas-filled polymers, in which hollow spherical filler particles are distributed, in comparison with traditional foams, have increased mechanical strength, electrical strength, resistance to climatic factors, less density difference and better surface quality. It is the advantages of syntactic foams listed above that make them relevant materials in the manufacture of structural elements of antennas. The purpose of the work is to study the possibility of obtaining products from low-density syntactic foam by combining an expandable polymer matrix with a filler in the form of hollow glass microspheres. The limits of filling the polymer matrix by the filler have been established. The material dielectric characteristics (dielectric constant ε and the tangent of the dielectric loss angle tgδ) and strength (compression stress at 10% deformation) characteristics have been investigated. The resistance of the material to the influence of climatic factors has been investigated. The technology of manufacturing the product by the flooding method has been developed. Also the effect of paint and varnish coatings on surface quality and dielectric characteristics of foams of low density has been investigated. The obtained results can be used to create structural elements of antennas.

Experimental Study on the Mechanical Properties and Microstructures of Cenosphere Concrete.

The most valuable components of coal fly ash are cenospheres. Cenospheres are hollow spherical particles produced during the coal-burning processes. As a result of their excellent characteristics, such as high workability, high heat resistance, low bulk density, and high strength, cenospheres can be used in the manufacturing of lightweight cement concrete. The research efforts and outcomes are to produce long-lasting cement-based lightweight concrete (LWC) composites with good mechanical properties. The novelty of this investigation is to determine the cement concrete strength when silica fume (SF) and cenospheres (CS) were used as a replacement for cement. Throughout the experiments, a consistent substitution of 12% silica fume was incorporated into cement mass. Silica is used as a micro filler and pozzolanic reactant to strengthen concrete. The concrete mixtures were tested to ensure they met the requirements of the lightweight concrete in terms of their mechanical, physical, and durability qualities. According to the findings, lightweight concrete standards were met, and environmental sustainability was improved with the use of these mix proportions. Concrete specimen's self-weight decreases by 35% with 30% cenosphere as a replacement. The micrograph shows the lack of portlandite is filled by mullite and other alumino silicates from the cenosphere. In order to achieve sustainability in concrete manufacturing, these mixtures can be suggested for the making of structural LWC that makes use of a large volume of industrial waste while conserving cement and natural resources.

Effect of Morphology on the In Vitro Bioactivity and Biocompatibility of Spray Pyrolyzed Bioactive Glass

Because of its fundamental characteristics, including bioactivity, biodegradability, and nontoxicity, bioactive glasses (BGs) become among the most impressive materials in the area of biomedical applications. It is primarily utilized in the applications of dermal fillers, orthopedic implants, and pharmaceutical delivery systems. Here in this study, simple and continuous methods were employed to produce hollow spherical bioactive glasses (HSBGs) microspheres. Using a spray pyrolysis method, solid and hollow spherical particles were successfully synthesized, and the particle formation mechanism was also discussed in detail. Surface and inner morphologies of synthesized bioactive glass (BG) powders were investigated using scanning electron microscopy (SEM) and transmission electron microscopy (TEM), respectively. BET was employed to measure the pore volume as well as specific surface area for pure BG and BG-derived-PEG powders; all p values are below 0.05, demonstrating noticeable distinction among both forms of synthesized powders. Using energy dispersive spectroscopy (EDS), the constituent components of prepared samples were assessed. In addition, by immersing samples in simulated bodily fluid (SBF) for 12 hr, in vitro bioactivity was tested by SEM. The viability of cells for both BG specimens was evaluated at various extraction concentrations by MTT assay (3-(4,5-dimethylthiazol-2-yl)-2,5-diphenylte-trazolium bromide) for 72 hr. Relying on the Ca/P ratio obtained from SEM-EDS, HSBGs possessed higher hydroxyapatite-forming capacity than solid bioactive glasses (SBGs). In addition, the cell viability of both specimens showed that at all extraction concentrations, they pass the standard biocompatibility levels. Therefore, the HSBGs have better biocompatibility and in vitro bioactivity; hence they are promising for future tissue-engineering development.

Polarized neutron scattering study of hollow Fe3O4 submicron spherical particles

We report results of polarized small-angle and wide-angle neutron scattering experiments at T = 10 and 300 K for 420 nm-sized hollow Fe3O4 spherical particles. Each hollow particle is a mesocrystal, which is composed of small nanoparticles with nearly the same crystallographic orientation. Polarized neutron experiments allow us to evaluate magnetic correlations of parallel and perpendicular magnetization components with respect to magnetic field during magnetization process. Small-angle neutron scattering reveals that as the magnetic field decreases from a saturation field of 10 kOe, the perpendicular magnetization component maximizes around zero applied field, whereas the parallel component minimizes. This behavior was observed below and above Verwey transition temperature of ∼ 120 K. Calculations of neutron intensities for vortex structures suggest the reorientation of the vortex core towards the magnetocrystalline anisotropy axis from the magnetic field direction at low applied fields. Moreover, a magnetic domain length obtained from the wide-angle scattering is of the order of 30-40 nm and comparable to the size of the small nanoparticles forming a hollow sphere, suggesting that magnetic correlations within the small nanoparticles always retain during magnetization process.

Production of functional spherical particles with porous hollow structures in water via oiling-out directional agglomeration

An efficient and green method for the production of porous hollow spherical particles was proposed by designing an oiling-out process, and functional spherical particles of indomethacin and nifedipine were successfully prepared in water.

Effect of PAA on the structure and transmittance of hollow spherical SiO2 film prepared by sol-gel method

Hollow spherical silica film has been used on glass substrates as an antireflective coating, and PAA is one of the common templates for preparing SiO2 films by sol-gel method. In this paper, the effects of polyacrylic acid (PAA) content on the SiO2 films prepared by the sol-gel method were analyzed using TEM, DLS, XRD, IR, SEM, and the transmission spectrum. The sol containing hollow spherical micelles can be prepared well from tetraethyl orthosilicate (TEOS) as raw material and PAA as water-soluble polymer by improved Stober, and template method. With the increase of PAA content, the particle size of hollow spherical silica colloidal particles increases and the quantity decreases. The sol is attached to the surface of the soda lime glass substrate to form a film by dip-coating method. With the increase of withdrawal speed, the transmittance of the glass increases, and with the increase of PAA content from 0.1g to 0.6g, the sample's transmittance decreases first and then increases.

Experimental study of the parametric impact on size growth of maltodextrin particles in counter-current spray dryer

Spray drying is closely related to the final product quality as it is mostly the last unit operation in the processing line. Counter-current spray drying experiments are relatively difficult to perform and due to this reason limited data is available. In this study, extensive experimental data is analyzed and results are presented. To better understand and optimize the performance of the spray dryer, an analysis of the final product received from 45 experimental runs of counter-current drying of maltodextrin solution was performed to check for the dependency of product properties on operating conditions. Effects of varying degrees of drying air swirl, atomizing nozzle gas-to-liquid ratios (GLR), the position of the nozzle with respect to drying air inlet, and power input for drying were analyzed. An optical measurement technique was used to measure the particle size distribution, velocity, and direction of motion. Samples were collected and scanning electron microscope (SEM) images were taken to analyze the particle morphology. Hollow spherical particles with thin crust were generated. Puffing and agglomeration were observed to be the phenomena that contributed to the increase in particle size, however the impact of puffing was found to be marginal. The results proved a strong dependency of the counter-current spray drying process on nozzle spray parameters and its position in the tower. The moisture content of the product showed a strong negative correlation with the temperature of the drying medium. • 45 experiments carried out in semi-industrial scale counter-current spray dryers using maltodextrin. • Impact of each operational parameter on product properties was calculated using the Pearson correlation factor. • Gas-to-Liquid ratio of the slurry at the nozzle had the highest impact on the final product. • Smaller droplets dry quicker and do not agglomerate as often as larger droplets. • Larger particles obtained had lower sphericity, indicating the formation of agglomerates.

Phase transformations and microstructure evolution during combustion of iron powder

To successfully transition from fossil-fuel to sustainable carbon-free energy carriers, a safe, stable and high-density energy storage technology is required. The combustion of iron powders seems very promising in this regard. Yet, little is known about their in-process morphological and microstructural evolution, which are critical features for the circularity of the concept, especially the subsequent reduction of the combusted oxide powders back to iron. Here, we investigated two iron powder combustion pathways, one in air and one with the assistance of a propane pilot flame. Both processes resulted in spherical hollow particles composed of a complex microstructure of wüstite, magnetite and/or hematite. Partial evaporation is indicated by the observation of nanoparticles on the micro-sized combustion products. The associated gas production inside the liquid droplet could be the origin of the internal porosity and micro-explosion events. Cracking at the end of the combustion process results in mostly open porosity, which is favorable for the subsequent reduction process. With this study, we aim to open the perspective of iron metal fuel from macroscopic combustion analysis towards a better understanding of the underlying microscopic thermodynamic, kinetic, microstructural and thermomechanical mechanisms.

Novel industrial waste-based shape-stabilized composite phase change materials with high heat storage performance from calcium carbide furnace dust

Phase change materials with stable shape and superb thermal properties play a significant role in efficient utilization of solar energy. Herein, a novel industrial waste-based shape-stabilized phase change material (SSPCM) with a high paraffin load (61.32%) and environment-friendly was designed and prepared via a vacuum impregnation method, in which calcium carbide furnace dust (CCFD) was selected as the supporting materials for the first time. The acidified CCFD (HCCFD) and activated HCCFD (ACCFD) were obtained by hydrochloric acid pickling and potassium hydroxide activation, respectively. Scanning electron microscope (SEM) showed that calcium carbide furnace dust contains a certain amount of hollow spherical particles, which provided a well-maintained skeleton for the phase change medium. Differential scanning calorimetry (DSC) results indicated that the Paraffin/ACCFD with 57.0% effective impregnation of paraffin. Besides, the maximum melting and freezing latent heats were around 106.31 J/g and 104.53 J/g, respectively. Moreover, the obtained SSPCM exhibited excellent chemical compatibility and thermochemical stability. It is worth noting that the Paraffin/ACCFD showed optimal thermal management ability. The current work suggested that the SSPCM prepared from calcium carbide furnace dust industrial waste has expansive application potentials in thermal energy storage and solar energy conversion.

Micro-Explosion mechanism of iron hybrid Methane-Air premixed flames

Compared to gaseous hydrocarbon flame, a metal hybrid flame has higher energy and even has a higher flame temperature. This study explored the micro-explosion mechanisms of iron particles in methane-air premixed flames. Four different particle sizes were used in this experiment, that is 2–10 μm, 50 μm, 100 μm, and 150 μm. A conical methane-air premixed flame was fixed in a stoichiometric condition and doped with iron particles at various feeding rates. The 50- and 100-μm iron particles had a 20% and 500% higher micro-explosion probability than the 2–10-μm particles. The micro-explosion probability would increase with the increase of particle size. In addition, the micro-explosion probability of the 2–10 μm iron particles was 125% higher under 10% CO addition than under no CO addition. CO plays a critical role in increasing the micro-explosion probability of iron particles smaller than 50 µm.Regarding the mechanism of the micro-explosion of iron particles, the oxidizer diffuses into iron particles and produces iron oxide in a thin shell. However, limited oxygen heterogeneously reacts over the surface of the iron particles and produces Fe3O4. Fe3O4 comprises FeO and Fe2O3. However, when CO reacts Fe3O4, this reaction makes Fe3O4 redox into fresh Fe particles, whereas it will lead to FeO while Fe reacts with O2 predominantly. The presence of FeO brings about iron particles blinking and shrinking right after passing the flame front. When CO was dominated in the reaction, it gave rise to the formation of Fe(CO)5 gas bubbles in the film of iron particles. During the agglomeration of the bubbles, incorporating Fe(CO)5, CO, O2, and N2, the iron particles expand gradually and become hollow spherical particles eventually. However, under special conditions, this particle expansion will deteriorate the micro-explosion of the iron particles.

Carbon‐containing droplet combustion–carbothermal synthesis of well‐distributed AlN nanometer powders

AbstractA novel three‐step technique was employed to synthesize the well‐distributed AlN nanopowders. First, the hollow spherical precursor particles with an average diameter of 2–5 μm, consisting of an amorphous structure mixture of Al2O3 and C, was prepared by carbon‐containing droplet combustion method by using glucose, urea, and aluminum nitrate as starting materials. The carbothermal reduction and nitridation (CRN) was carried out at 1500°C under N2 flow for 2 h and subsequently the CRN product was calcined at 700°C in air for 1 h to remove residual carbon and transform the CRN product to high‐purity AlN powders consisting of nanostructured hollow spheres. The formation mechanism of precursor and AlN hollow spheres was discussed in detail. The AlN powder exhibited well‐distributed spherical particles with a size of 30–50 nm and good sinterability. After additive‐free and pressureless sintering at 1800°C for 2 h, the relative density of the sintered AlN sample was measured to be 99.02%.